The present invention relates to tools for cutting hard, non-metallic materials including abrasive wood and wood-based composites. More specifically, tools of interest include circular saws, milling cutters, routers, panel cutters and similar tools whose cutting edges can be fabricated from blanks of ultrahard polycrystalline cubic boron nitride (CBN) or the like.
Background: Woodworking Tools
Tooling for woodworking-type applications has some significant differences from the requirements of metalworking. (Many of the materials which are cut in woodworking-type applications are not merely wood, and sometimes not wood at all: particleboard and oriented-strand fiberboard, as well as non-wood polymers such as Melamine(trademark) or other inorganic-loaded durable composites, may be encountered.) Common features of woodworking-type applications include air cooling (and associated high tooth speeds), workpiece materials with shear strengths much lower than ferrous metals, high shock loading (in many cases), and high abrasion. (Even among pure wood materials, many include microparticles of silicon dioxide, and composite materials may contain very abrasive filler components.)
Background: Carbide-Toothed Circular Saws
Cutting tools (especially woodworking tools) often use inserted teeth of a material which is harder than the hardest of steels. The most common material used for this is a xe2x80x9ccemented carbide,xe2x80x9d which typically includes small grains of tungsten carbide bonded into a matrix with a metal (typically cobalt). (Because the strength and hardness of the matrix are derived from the grains of tungsten carbide, such cemented carbides are often referred to simply as xe2x80x9ccarbide.xe2x80x9d) Such xe2x80x9ccarbidexe2x80x9d saw tips have a hardness of about 92 (Rockwell A).
Some firms manufacture only the steel bodies of circular saws, which are hardened, tempered and finished in every way except for tipping, and are then sold to other saw manufactures who specialize in carbide tipping. Other firms manufacture the complete saws including both the steel bodies and the installed tips. In either case, the same standard carbide tips are used in the fabrication of the blades. The steel bodies are normally made of high-carbon alloy tool steel, then a pocket is ground into the periphery of the saw body to accommodate the carbide tips. The tips may be xc2xc to xe2x85x9c inches long, 0.062 to 0.093 inches thick and from 0.10 to 0.375 inches wide, depending on the width of the finished saw blade.
In the woodworking industry, carbide tipped saws are typically 8 to 20 inches in diameter. Depending on their function, the 8 inch blades may have between 24 and 48 teeth, and the larger saws 60 to 100 teeth. For cutting non-ferrous metals, the number of teeth is typically between 24 and 80 for saws ranging from 8 to 18 inches in diameter. However, saws with greater tooth density (i.e. more teeth per inch) would be required to produce superior finishes and to cut thin materials.
Background: Ultrahard Cutting Tool Materials
Carbides were invented in the 1920s, and the search for better cutting materials continues to this day. In general, the ideal cutting tool surface should combine abrasion-resistance (hardness) with shock-resistance (toughness). (Of course there are many other relevant properties, including yield strength, rigidity, temperature limits, corrosion resistance in some applications, etc.) Materials which are harder than carbides are particularly interesting for woodworking applications, as well as many other applications.
In early 1970s, General Electric Company introduced a variety of Polycrystalline Diamond (PCD) cutting tool materials consisting of a layer of micron-sized diamonds integrally bonded with a carbide substrate. These man-made ultrahard crystalline and polycrystalline compounds have become readily available from commercial sources in a variety of grades, making possible tremendous advances in cutting tool design.
In practice, thin layers of PCD or CBN are bonded to a disk of tungsten carbide substrate ranging from 60 to 100 mm in diameter. The process requirements are extreme, e.g. 1300xc2x0 C. and tens of thousands of atmospheres of pressure. These bonded disks, or wafers, generally have a combined thickness of around 3 to 4 mm with PCD or PCBN forming a single-sided layer 0.1 to 0.3 mm thick. The substrate face of tungsten carbide is ground flat and overall thickness is further reduced by grinding to one of several industry standard dimensions.
Then, using sophisticated computer controlled wire electrical discharge machine tools, the wafers are sliced into squares, rectangle, and round shapes dimensionally similar to standard carbide blanks and inserts. Ultimately, these xe2x80x9cpreformsxe2x80x9d are ground into final dimensions for lathe tools or otherwise incorporated onto tool steel bodies in the same manner as carbide tips and inserts, and are sharpened by various special techniques.
The diamond layer""s abrasion resistance, coupled with the carbide""s strength, produced a cutting tool material that achieved a tremendous increase in machining performance over other available materials, tungsten carbide, for example. PCD is primarily used in non-ferrous metalworking applications such as copper and aluminum or to machine plastics, rubber, synthetics, and laminates. It had also found widespread use in sawing and shaping medium-density fiberboard and chipboard in the furniture industry. Unfortunately, notwithstanding is superb properties, it reacts chemically with iron and steel and cannot be used to machine any steel alloy.
Polycrystalline Cubic Boron Nitride (PCBN) is used for machining ferrous materials such as gray cast iron. PCBN is manufactured like PCD, except that a layer of cubic boron nitride crystals replace the diamond. Excellent machining results are obtained with PCBN-based tools in finish-turning work on nickel-based alloys. Because of its great hardness and wear resistance, PCBN cutting tools can be used at high cutting speeds and temperatures. In addition to higher available cutting speeds and excellent wear behavior, PCBN cutting materials achieve longer tool lives, allowing parts to be finished in a single cut, reliably attaining high accuracy over a long machining time.
Both PCD and PCBN provide major improvements over conventional carbide cermets, and it is now possible to machine substances that have previously been extremely difficult to fabricate. The most common ultrahard materials used in modern tools are polycrystalline diamond (PCD), which is 3.6 times harder than tungsten carbide, and cubic boron nitride (CBN), which is 2.8 times harder than carbide. However, the very properties of hardness and abrasion resistance that make polycrystalline tools superior cutting devices also make these tools extremely difficult to grind and finish.
Background: Cost Considerations for Ultrahard Materials
Despite their extraordinary performance, the application of these ultrahard materials is frequently limited by their high cost, which is at least ten times that of tungsten carbide. In addition, because of their extreme hardness, they can only be shaped with varying degrees of difficulty. PCD can only be ground by special diamond grinding wheels that are no harder than the PCD, and therefore, have a short service life. Other means of shaping PCD include electrodischarge machining (EDM) by either wire or shaped carbon electrode methods. Both of these methods require expensive, specialized computer controlled equipment that further adds to the cost of the tools in which they are incorporated.
The cost of polycrystalline diamond (PCD) and cubic boron nitride (CBN) are approximately the same. One might think, therefore, that absent diamond""s inability to machine ferrous materials, there would be no practical use for PCBN which is less hard and less resistant to abrasion than PCB. Presumably because of the technical superiority of PCD over PCBN, no manufacturer recommends PCBN for wood, wood-composite products or plastics. Further, no toolmaker supplies tools for these applications.
Background: Grit-Surfaced (Non-Toothed) xe2x80x9cSawsxe2x80x9d
A common type of cutting tool is a circular blade which does not have shaped teeth at its edge, but which is simply coated with a diamond grit. Such cutting tools are commonly referred to as diamond xe2x80x9csaws,xe2x80x9d but in fact they do not perform the same type of material-removal action as is performed by a saw with shaped teeth. A saw with shaped teeth, when it is operating correctly, will carve off chips of material. By contrast, a grit-coated blade will have more of a scraping or abrasive action. (See generally Jim Effner, Chisels on a wheel (1992); and Peter Koch, Utilization of Hardwoods Growing on Southern Pine Sites (1985); both of which are hereby incorporated by reference.) A cutting action is greatly preferable for many applications, to produce a cleaner cut, lower temperature, and lower power requirements.
Polycrystalline Cubic Boron Nitride (PCBN) Woodworking Tools and Methods
The present inventors have discovered that PCBN cutting tips can be accurately ground with the same equipment commonly used to fabricate high quality tungsten carbide tools, with substantially the same geometries, and with only slight modifications of technique. Thus it turns out that, for woodworking applications, PCBN tooling is much more nearly analogous to carbide than to diamond. This is quite contrary to common belief in the industry, and radically changes the economics of PCBN tooling.
There are severe restrictions on tooth geometry of PCD tools, particularly the hook angle: the use of positive hook angles (as is usual with circular saws for woodworking) can cause PCD tools to chatter or to suffer fracture. (Hook angle is the angle of the leading face of the tooth: if the tooth is angled to pull workpiece material back toward the center of the blade, it is said to have a positive hook angle.) Thus use of very small or negative hook angles is necessary with PCD tools. The geometry of PCBN cutters however, can be made to very closely approximate those of proven carbide tools, i.e. positive hook angles can be used for faster and cooler cutting.
A profound advantage of PCBN over PCD in all but the largest operations, is that PCBN tools can be maintained using modified $20,000 grinding machines where PCD requires an electrodischarge machine costing ten times as much. This makes on-site or near site service feasible, reduces tool repair costs, turnaround time, and the inventory cost of spares.